1. Field of the Invention
The present invention relates to a steering angle control apparatus for steering the vehicle wheels according to the steering wheel angle, vehicle speed, vehicle yaw rate, and rear wheel steering angle of the automobile or other vehicle.
2. Description of the Prior Art
Numerous four-wheel steering control apparatuses have been proposed for improving the steering stability of a vehicle in motion. With the apparatus disclosed in Japanese Laid-open Patent Publication No. H3-164374 (unexamined) published Jul. 16, 1991, for example, the steering wheel angle is detected by means of a steering wheel angle sensor. The actual yaw rate (the rotational angle velocity around the vehicle's center of gravity as seen from above) is detected by means of a yaw rate sensor. The steering speed of the front wheels is multiplied by the amount of steering control applied in the direction constraining the yaw rate (the yaw rate constraining direction). The amount of steering control applied in the yaw rate constraining direction is then adjusted according to the steering speed, thereby reducing the amount of steering control applied in the yaw rate constraining direction of the rear wheels, which are in steady rotation, and improving the initial turning response when the front wheels are suddenly turned.
The yaw rate tracking control method proposed in Japanese Laid-open Patent Publication No. 60-124572 (unexamined) published Jul. 3, 1985, for example, calculates a target yaw rate from the steering wheel angle and vehicle speed, and steers the rear wheels so that the actual yaw rate tracks the target yaw rate.
By using yaw rate feedback, both of these methods provide the advantage of being able to compensate for deviations in the orientation and line of travel of the vehicle due to external disturbances such as cross winds and poor road conditions by steering the rear wheels.
However, when drift affects the output of the yaw rate sensor, yaw rate sensor output deviates from the actual yaw rate by the amount of drift. As a result, the steering angle of the rear wheels may be erroneously changed, even though the vehicle is travelling straight and the front steering angle is zero degrees, because of a false yaw rate sensor output.
When the road surface is inclined on curves (banked) or straight roads (canted), gravity is a steady disturbance affecting the motion characteristics of the vehicle, and the vehicle behaves differently than when on flat roads.
More specifically, when moving through a bank, the yaw rate remains constant irrespective of the steering wheel angle if the vehicle speed remains constant.
When travelling on a canted surface, the front wheels must be turned at an angle merely to maintain a straight line of travel.
If a target value achieving a vehicle sideslip angle of zero on a flat road is determined from the vehicle speed and steering wheel angle, and the wheels are controlled using a yaw rate tracking control method that tracks this angle, the following phenomena resulting in a sense of instability occur.
On banks, the target yaw rate obtained from the steering wheel angle will differ from the yaw rate needed to travel in a stable manner through a bank.
However, because this control method tracks the actual yaw rate to the target yaw rate, the yaw rate of the vehicle does not match the yaw rate required for the bank, and the driver must steer to compensate for the difference.
On canted roads, the target yaw rate is calculated to be in the direction in which the steering wheel is turned, and the rear wheels are turned to track the actual yaw rate to the target.
However, because the front wheels must be turned for the vehicle to travel in a straight line on a canted road, this turning of the rear wheels hinders driving in a straight line, and the driver must again steer to compensate for the difference.
In FIG. 22, one example of the conventional steering angle control apparatus described above, applied in a vehicle V having front wheels 20 and rear wheels 29, is schematically shown. The steering angle control apparatus includes a vehicle speed detector 11, a steering angle detector 12, a yaw rate detector 13, a desirable yaw rate calculator 14, a desirable steering angle calculator 17, and a rear wheel steering device 40. The desirable yaw rate calculator 14 calculates a target yaw rate and produces a desirable yaw rate signal SYm for the vehicle Vc based on a speed signal Sv obtained by the vehicle speed detector 11, and a steering angle signal Ss obtained by the steering angle detector 12. The desirable steering angle calculator 17 calculates a desirable real wheel steering angle for the rear wheel and produces a desirable rear wheel steering angle signal .theta.r based on the desirable yaw rate signal Sym. According to thus obtained desirable rear wheel steering angle signal .theta.r, the rear wheel steering device 40 steers the rear wheels 19.
As a result, due to the effects of sensor drift and gravity, steady disturbances that are undesirable when calculating the rear wheel steering control tend to bias the detected yaw rate, and must therefore be removed.